The invention relates to a method for producing image contrast by phase shifting in electron optics, wherein, starting from an intermediate image, an anamorphic image of the axial rays is produced by quadrupole fields with simultaneous passage through zero of the field rays in at least one diffraction intermediate image plane, where a relative phase shift between a region around the electron beam of zeroth order of diffraction and the electron beams of higher orders of diffraction is caused by a magnetic or electric field, and thereafter the at least one anamorphosis of the beam path produced is corrected again by further quadrupole fields.
The invention also relates to a device for performing the method with entry side quadrupole elements that, from an intermediate image, produce quadrupole fields such that the axial rays and the field rays are focused and defocused in two perpendicular sections in such a way that, in a diffraction intermediate image plane, the axial rays form an anamorphic image and the field rays each pass through zero, wherein, in the region of the diffraction intermediate image plane, a central quadrupole element is disposed with a quadrupole field such that the axial rays exit the field with slopes opposite to those for the entry into the field, and wherein two further quadrupole elements produce quadrupole fields that have the same magnitude and sign as the quadrupole fields of the quadrupole elements on the entry side, so that the anamorphosis of the beam path is corrected again, and wherein, in the diffraction intermediate image plane, a phase-shifting element is disposed whose magnetic or electric field is arranged in such a way that a relative phase shift is caused between the region of the electron beam of zeroth order of diffraction and the electron beams of higher orders of diffraction.
The invention also relates to a device for performing a method with entry-side quadrupole elements that, from an intermediate image, produce quadrupole fields such that the axial rays and the field rays are focused and defocused in two perpendicular sections in such a way that, in a diffraction intermediate image plane, the axial rays form an anamorphic image and the field rays each pass through zero, wherein, in the region of the diffraction intermediate image plane, a central quadrupole element is disposed with a quadrupole field such that the axial rays exit the field with slopes opposite to those for entry into the field, and wherein, in the diffraction intermediate image plane, a first phase-shifting element is disposed whose magnetic or electric field is arranged in such a way that a relative phase shift is caused between the region of the electron beam of zeroth order of diffraction and the electron beams of higher orders of diffraction.
As in light microscopy, in electron microscopy many specimens are almost transparent to the beam so that only very little amplitude contrast is achieved with conventional imaging. However, both the axial ray and the field rays are subject to a local phase shift that depends on the structure of the specimen on passing through the specimen. This splits the beam into a non-diffracted zero beam and diffracted beams of multiple orders of diffraction in the specimen.
If a phase shift is imposed on the zero beam, preferably 90°, the phase modulation of the specimen is converted to a strong amplitude contrast when the diffracted beams are again superimposed on the zero beam in the image plane. This is the known phase contrast (see, for example, Reimer, Kohl, “Transmission Electron Microscopy,” p. 211 ff, 5th Edition, 2008). Such a phase shift is achieved using the fact that beams form different focal points in the focal plane of the objective lens. The zero beam thus forms a central focal point in the focal plane for the zero beam and is surrounded by the focal points of the diffracted beams of the various orders of diffraction, starting with the diffraction of the first order, which are essentially all in the same focal plane. This fact can be used to cause a phase shift either of the zero beam or of the diffracted beams to cancel out the phase difference, thus amplifying the amplitudes.
Unlike in light optics, however, with electron beams there is the problem that there is no transparent plate for them that could hold a phase plate. To solve this problem, phase-shifting elements of many different types have therefore been suggested based on the fact that the phase-shifting element reaches right through the region of the diffracted beams to influence the zero beam. It must be noted that it is not technically feasible to influence only the zero beam because this would require an electric field in the nanometer range, which cannot be produced with current technical possibilities. For that reason, depending on the nature of the phase-shifting elements used, diffracted beams are also affected at least to a small degree. In many of these suggestions, the phase-shifting element has an annular structure that includes the region of the zero beam in order to apply the field required to phase-shift the zero beam there. However, this shades diffracted beams to a not inconsiderable degree, which are then not available for contrast production. For that reason, DE 10 2006 055 510 A1 proposed a phase-shifting element that extends on one side only up to the region of the zero beam to apply the necessary field there. Because, in this way, no regions are shaded that are mutually offset by 180°, with this phase-shifting element it is possible to reconstruct the shaded portions of the diffracted beams and fully use them for contrast production.
Another solution to this problem are phase-shifting elements of the type stated above that were proposed by DE 10 2007 007 923 A1. They work according to the method stated above based on the fact that an anamorphic image is produced in a diffraction intermediate image by quadrupole fields, which make it possible to reach the zero beam or the diffracted beams with a field in order to cause a relative phase shift between the region around the electron beam of zeroth order of diffraction and the electron beams of higher orders of diffraction. In this way, no component of the device shades diffracted beams. This has the further advantage that separate influencing of these beam components and therefore contrast production is considerably improved. The zero beam and diffracted beams are sequenced from the center outward in the anamorphic image so that it is possible to affect both the zero beam and the diffracted beams relatively precisely with one field. However, it is not possible to extend the length of the anamorphic image to an unlimited degree to achieve good influencing of either the zero beam or the diffracted beams because such an extension is limited both technically by the size of the tube of the electron microscope containing the electron-optical system and by the fact that extending the length of the anamorphic image beyond a certain point results in errors that are no longer tolerable, thus forfeiting the desired high image quality again.
The object of this invention is therefore to improve the image contrast in the method and the two devices stated above without causing errors that are no longer tolerable.